It is well-known that the temperature of the active layers of a semiconductor laser must be closely controlled to operate the laser within close tolerances for both wavelength and power. A so-called ThermoElectric Cooler (TEC) is often used for this purpose, since a TEC can be controlled to either add or extract heat from a laser depending on whether the desired operating temperature is above or below the ambient temperature. However, the amount of power used to cool a high power laser to a desired temperature could exceed the amount of power that is used to drive the laser. This could lead to thermal runaway, since the additional power supplied to cool the laser may increase the temperature of the operating environment of the laser, rather than decreasing it.
A number of controllers have been developed to control the operation of a TEC, and thus the cooling/heating of a semiconductor laser. Included among these prior art controllers is the so-called proportional controller illustrated in FIG. 1. The proportional controller uses two power supplies, 5-1 and 5-2, which output voltages of opposite polarities but nomially of equal levels, e.g., .+-.5 volts, which are supplied to TEC 6 via transistors Q6 and Q8, respectively, under control of amplifier 4. Transistors Q6 and Q8 are controlled by amplifier 4, which is connected to TEC 6 via feedback loop 6-1. The voltage level in the feedback loop changes as the resistance of a thermister in TEC 6 changes, which indicates the current level of cooling at TEC 6. That is, the TEC thermister is connected to a conventional balanced bridge circuit in the RF filter and amplifier circuit. The amplifier circuit amplifies an error signal that is developed across the bridge when the bridge is not balanced, and uses the amplified signal to "turn on" either transistor Q6 or Q8 a certain amount to respectively increase the cooling or heating that TEC 6 is supplying to the laser (not shown).
Disadvantageously, we have recognized that the control arrangement of FIG. 1 is inefficient, since each of the power supplies 5-1 and 5-2 have to operate at full voltage, even when only a fraction of that voltage level is being supplied to TEC 6. For example, assume that each power supply needs to be at 5 volts to provide maximum cooing. If TEC 6 currently needs only 1 volt of cooling, then the remaining 4 volts would be dissipated across the respective one of the transistors Q6 or Q8 that is currently delivering the 1 volt to TEC 6.
Another prior art controller which uses just one power supply is shown in FIG. 2. Power supply 9, more particularly, supplies a voltage signal, +TEC, and RTN (return) signal, to the TEC and RTN inputs of conventional H-bridge 10. H-bridge 10 switches its inputs (+TEC and RTN) to its TECA and TECB outputs based on a set of logic signals that it receives via logic inputs COOL and HEAT. For example, if the input designated Heat is at a first logic level, e.g., ground/return, and the input designated COOL is at an second logic level, e.g., +5 volts, then H-bridge 10 switches its +TEC input to its TECA output and switches its RTN input to its TECB output, which are directly supplied to TEC 12 and which causes TEC 12 to enter a heating cycle. For the reverse case, in which the input designated HEAT is at the second logic level and the input designated COOL is at the first logic level, then H-bridge 12 switches its TEC input to its TECB output and switches its RTN input to its TECA output, which causes TEC 12 to enter a cooling cycle. Similarly, the TEC 12 thermister serves as an element in a balanced bridge within RF filter and amplifier circuit 7 to control the amount of cooling/heating that is supplied to a system laser (not shown). Circuit 7 amplifies the signal that the bridge outputs, and supplies the amplified result to comparator 8. Comparator 8, in turn, compares the amplified signal with a control signal indicative of the desired level of such cooling to determine if the level of the amplified signal is above (cool) or below (heat) the level of the control signal, and then adjusts the COOL and HEAT logic levels accordingly.
We have recognized that the controller of FIG. 2, for the most part, continuously alternates between a cooling cycle and heating cycle. The reason for this is that power supply 9 continuously supplies a voltage level that provides maximum cooling (or heating). That is, when the latter voltage level is supplied to TEC 12, TEC 12 cools the semiconductor laser toward a maximum level. Since there is a delay between the time such cooling occurs and the time that the thermister signal reaches a level indicative thereof, the cooling that TEC 12 provides significantly overshoots the target level. At that point, amplifier 7 and comparator 8 invoke a heating cycle to drive the overcooling of the laser to the target temperature. The heating cycle similarly overshoots the target level which drives the level of heating at the laser beyond the desired level. At that point amplifier 7 and comparator 8 invoke a cooling cycle, and so on. The power expended to alternately provide such heating and cooling can result in the loss of an appreciable level of power as is illustrated in FIG. 3 by the shaded portions of the power diagram.